Thanks to Ozh for his WordPress Avatar Plugin (Version 1.1) which I just installed.
I edited the location of the default gravatar image in the wp_ozh_avatarpopup.php file, then uploaded it to the Plugins directory and activated it.
Basically, these are the options available (minus the spaces!!), check Ozh’s page for examples:
EDIT (27/7/07): I have deleted this – I wasn’t really using it, and keeping it meant I just had to remember another thing. 😀